Centralizer

ABSTRACT

A centralizer  10  for mounting on a tubular member such as a section of casing, together with one or more stops  42, 48  for retaining the centralizer thereon. The stops  42, 48  are provided with profiles for engaging the centralizer to restrict rotation thereof. The centralizer  10  may be selectively permitted or restricted from rotation by selection of appropriate stops, and by relative movement of the stops and centralizer on the tubular. Certain embodiments of the centralizer  10  may also include blades  14  on the body thereof, the blades being formed such that the velocity and kinetic energy of fluid flow across the blades is altered, so reducing settling of drill cuttings within the bore and on the centralizer.

FIELD OF THE INVENTION

This invention relates to a centraliser for use when running tubularsinto a drilled bore, and to other items that may be utilised inconjunction with centralisers.

BACKGROUND OF THE INVENTION

In the oil and gas exploration and production industry, subsurfacehydrocarbon-bearing rock formations are accessed by bores drilled fromsurface. The drilled bores are lined with tubular members,conventionally metal tubing known as casing or liner; for brevity,reference will be made primarily herein to casing. The casing istypically cemented in the bore by passing a cement slurry up between theannulus between the casing and the bore wall.

Any drilled bore will typically extend through a variety of formationtypes having different properties, for example formations which mayswell after drilling to restrict the bore diameter, due to the chemicalinteraction with the drilling fluid. Also, when drilling betweenformations of different hardness, it is common for a ledge to be createdat the transition between the formations. Further, in deviated orhorizontal bores, drill cuttings can fall out of mud suspension and willoften collect on the low side of the bore, to form cuttings beds. If thecuttings are not cleaned from the bore, the presence of the cuttingsmakes successful running in and cementing of casing more difficult andin some cases impossible. In an effort to overcome these difficulties, alength of casing may be provided with a shoe at its leading end, whichshoe may include numerous features, including cutting blades, aneccentric or offset nose, jetting ports and like, all intended tofacilitate progress of the casing past obstructions in the bore. Tominimise the drag between the casing and the bore wall as the casing isrun into the bore, and also to facilitate rotation of the casings as itis run in and during cementing, casing strings are often provided withcentralisers at various points along the length of the string.Centralisers are conventionally annular, to permit mounting on thecasing, and feature upstanding spaced apart blades which allow fluid andcement passage.

It is among the objectives of the various aspects of the presentinvention to provide centralisers and other apparatus to facilitate therunning in and cementing of casing and other tubulars.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided acentraliser comprising a body adapted for mounting on a tubular memberand defining a plurality of blades, the body being selectively bothrotatable and non-rotatable about the member.

This aspect of the invention permits the centraliser to rotate about thetubular member, which may be casing, in situations where thisfacilitates movement of the casing in a bore. However, if required, thecentraliser may be configured to rotate with the casing, which may beuseful if the casing is being moved through a bore restriction, and theblades of the centraliser may be utilised to ream or dislodge therestriction.

The invention also relates to a method of running a tubular member intoa bore, the method comprising providing a bladed centraliser on themember, and selectively coupling the centraliser to the member androtating the centraliser with the member.

In certain embodiments of the invention, the centraliser is adapted formounting towards the lower end of a string of tubular members. Otherembodiments may not be so adapted, depending on the operation for whichthe invention is to be used, and whether the invention is used withcasing or liner.

The centraliser may be provided in combination with a stop or otherengagement member for mounting on the tubular member, where the stop isadapted to cooperate with the centraliser to permit or restrict relativerotation between the centraliser and the tubular member. Conveniently,the stop is adapted to be fixed relative to the tubular member and thecentraliser is adapted to be normally rotatable relative to the member.Preferably, the stop and centraliser define cooperating formations whichmay selectively engage to restrict rotation therebetween. Mostpreferably, the centraliser is adapted to be axially movable, at leastto a limited extent, relative to the tubular member, and is axiallymovable into and out of engagement with the stop. The stop andcentraliser may be adapted and located such that axial movement of thetubular member through a bore in one direction will tend to separate thestop and centraliser, permitting rotation of the centraliser relative tothe member, while movement of the member in the opposite direction willtend to bring the stop and centraliser together, such that thecentraliser may be rotated with the member. Thus, the centraliser may berotatable on the member as the member is run into a bore, to minimisetorque and drag on the advancing and possibly rotating member, but maybe rotatable with the member as the member is pulled and rotated fromthe bore (a process known as ‘back reaming’). Such rotation of thecentraliser may assist in dislodging drill cuttings and obstructions, tofacilitate fluid circulation and tubular member movement once running inis recommenced. Thus, if there are difficulties encountered in fluidcirculation while running the member in, the member may be pulled back asufficient distance to engage the centraliser and stop, and the memberand centraliser then rotated to clear the obstruction to circulation.Alternatively, the centraliser and stop may be arranged such that thecentraliser is rotatable with the member as the member is run into abore, but is rotatable on the member as the member is pulled orretrieved from the bore. Further, the centraliser may be provided incombination with two stops, the centraliser being provided on the memberbetween the stops and being configured such that the centraliser isselectively rotatable with the member while the member is being run intothe bore and also while being retrieved or pulled from the bore.

Either or both of the stops may be reversibly mounted on the member,with a first end of the stop carrying a means for selectively engagingthe centraliser such that the centraliser is rotated with the memberwhile a second end does not. This allows the stop to be fitted in eitherorientation, depending on whether it will be desired during a downholeoperation to engage and rotate the centraliser, or whether it will bedesired to prevent such engagement and rotation occurring.

The centraliser may be adapted to be non-rotatable relative to themember on experiencing an axial force in excess of a predeterminedlevel, for example on the centraliser encountering an obstruction orrestriction which is not initially dislodged or negotiated by axialmovement of the centraliser, the centraliser may be pushed intoengagement with a cooperating profile or formation on the member, mostpreferably provided by a stop, which causes the centraliser to rotatewith the member and assists in dislodging or otherwise removing ornegotiating the obstruction or restriction.

The centraliser which is non-rotatable relative to the member may beadapted to be rotatable relative to the member on experiencing a torque,load, or force above a predetermined level. Thus, if the centraliserencounters a restriction or obstruction which is not overcome or removedby the rotating centraliser, the centraliser may rotate to avoid thetubular member experiencing excessive and potentially damaging forces.This may be achieved by providing a cooperating profile or formation onthe member, most preferably provided by a stop, which will disengage onexperiencing a predetermined torque. This may be achieved by providingcooperating teeth or the like adapted to ride over one another, a sprungretainer, or a “one-off” release, such as a shear retainer between thestop and the member, or forming a profile from deformable material.

The centraliser blades may take any appropriate configuration to providea stand-off between the tubular member and the bore wall and permitfluid circulation past the centraliser. The blades may be helical orextend substantially axially or circumferentially, or may be in the formof discrete protrusions or studs. The blades may be continuous ordiscontinuous, the latter arrangement being preferred to facilitatefluid and cement flow. The blades may be of similar configuration overthe length of the centraliser or may vary, and the centraliser may besymmetrical or non-symmetrical. The height of the blades may vary, andthe variation may be between circumferentially spaced blades or betweenaxially spaced blades. The height of each individual blade may vary,either continuously or in a stepwise manner. The blades may be providedwith cutting edges. In order to promote hole cleaning, the centralisermay be configured such that the centraliser has substantially completecircumferential blade coverage about its horizontal axis.

The blades are preferably separated by flutes, where the flutes may beof substantially constant cross section or which may define a varyingcross section, for example the flutes may define a venturi form, toaccelerate fluid flow therethrough and facilitate cuttings entrainment,or may be of substantially constant area but vary in form, for examplechanging from a relatively narrow and deep form to a relatively shallowand wide form to direct a greater proportion of the flow along the borewall.

Preferably at least one of the blades and flutes are configured to causea change in fluid velocity, pressure, or flow direction as fluid passesover or through the centraliser. Preferably the blades and flutes areconfigured to cause fluid velocity or speed to increase as fluid flowsbetween the blades, and to cause fluid velocity or speed to drop asfluid flows beyond the blades. This change in speed or velocity causesthe fluid flow to be turbulent, which in turn reduces the build up ofparticulates and the like around the centraliser and in the bore.

Preferably, the blades and/or flutes are configured to provide arotational force to the centraliser as fluid passes between the blades.This causes the centraliser to rotate, in the absence of anycountervailing force, which serves to entrain cuttings and particulatesin the fluid flow, and to prevent settling of cuttings, so reducing thebuild up of particulates and the like around the centraliser and thebore.

Preferably, the centraliser comprises a body on which the blades aremounted or formed. The body may be in one or more parts and may be ofany appropriate material. A bearing may be provided for engaging thetubular element, preferably the bearing being formed to encourage thinfilm lubrication or formation of a hydrodynamic bearing, and preferablyto provide sacrificial self-lubrication in the event that thin filmlubrication or hydrodynamic bearing should break down. The bearing maybe of the same or different material from the remainder of the body, andmay be integral with the remainder of the body or may be provided as aseparate part. The bearing may be a sleeve or the like or may provide adiscontinuous contact with the tubular member, for example the body maydefine a number of apertures in which plastics bearing inserts areprovided. The blades may also be of the same or different material asthe body. In one embodiment the blades are formed of a sacrificialself-lubricating material, such as a high performance plastic, tominimise friction between the centraliser and the bore wall. The bodymay be formed of a more rigid material, such as a metal, adapted toreceive the blades. The blades may be moulded into or otherwise fixed tothe body, for example the body may define slots or channels forreceiving the blades, which may be fixed to the body by means of a forcefit, by adhesive, or by fixings such as screws, bolts or dowels. Thebody or bearing may be of plastics or metal, including aluminium,aluminium alloy, aluminium bronze, phosphor bronze, cupro-nickel, zincalloy, brass, copper alloys including gun metal, steel, iron, ironalloy, austempered ductile iron, AB2, phenolic resin, thermoplastics,PPP6, PPP12, PEEK, Nylon® 6.6 polymer Nylon® PA12G polymer, or “V” gradeplastic manufactured by Devol Engineering Ltd.

Alternatively, the body or bearing may be formed of one of thesematerials or from carbon reinforced polyetheretherketone,polytetrafluoroethylene, polyphthalamide, or polyvinylidene fluoridecompounds.

Where formed of metal, the body or bearing may be coated withpolytetrafluoroethylene (PTFE), electroless nickel, zinc, paints andplastics including: carbon reinforced polyetheretherketone;polyphthalamide; polyvinylidene fluoride compounds; phenolic resins orcompounds; thermosetting plastics; thermoplastic elastomers;thermoplastic compounds; thermoplastics including polyetheretherketone,polyphenylenesulfide, polyphthalamide, polyetherimide, polysulphone,polyethersulphone, all polyimides, all polyamides (including nyloncompounds), polybutyleneterephthalate, polyetherketoneketone.

Where appropriate the body or bearing material may contain anappropriate filler, such as glass, carbon, PTFE, silicon, molybdenumdisulphide, graphite, oil and wax.

Where appropriate the body may be in the form of a frame or cage ofharder material (such as metal) on or around which is provided a portionor portions of softer material (such as plastics). This provides somereinforcement to the body to resists stresses. The frame may be in theform of a solid cylinder, or be provided with holes or cutouts, or be inthe form of a mesh or network.

The body may be of unitary construction, or may be formed of two or moreparts to allow the body to be fitted around a tubular. The ports may bejoined by any convenient means, for example a hinge and pin, the portsmay snap-fit together, or the ports may be profiled so that they may beslid together.

The centraliser may be provided in combination with one or more stopsfor mounting on the tubular member, the stops at least limiting axialmovement of the centraliser relative to the member. The stops may bemounted on the tubular member in any appropriate manner, however it ispreferred that the stop comprises at least two parts, and that when theparts are coupled together a portion of at least one part is urged intoengagement with the tubular member. Most preferably, one part defines amale part and the other part defines a female part, the male part beingdeformable so that it may be urged to assume a smaller diameter on beingcoupled with the female part. The male part may be slotted or otherwiseformed to facilitate deformation.

In another embodiment the stop comprises a body and a radially movablegripping part for selectively engaging the tubular member, and means forurging the gripping part into engagement with the tubular member. Thegripping part is preferably in the form of a split ring, and the urgingmeans is in the form of one or more screws or bolts mounted in the body.The gripping part may comprise a high-friction surface, such asaggregate or serrated grooves, to increase the effectiveness of thegripping.

The stop preferably has a tapering leading face, to facilitate movementover ledges and the like and to prevent the build up of cuttings andother debris in front of the stop.

In one embodiment of an aspect of the invention, a centraliser comprisesa similar arrangement for securing the centraliser to a tubular member.Conveniently, screws or bolts provided to urge the gripping part intoengagement with the tubular member are accommodated in raised or upsetportions of the centraliser forming blades or pads of the centraliser.

According to a further aspect of the present invention, there isprovided a guide shoe for mounting on the end of a tubular member, theshoe comprising a body having a bore formed therethrough leading to anopening, the opening being in the form of a slot.

A shoe of the present invention may be mounted to the end of a casingstring, where the bore and slot allow fluid to be passed through andthen exit the shoe to dislodge and entrain cutting waste and the like.The slot formation of the opening causes the fluid flow to extend over agreater length than conventional jetting ports; if a section of the slotshould become blocked by for example cuttings, fluid may stilt flowthrough the remainder of the slot and act upon the blockage to clear it.Thus, the present invention reduces the likelihood of the openingbecoming clogged.

Preferably the shoe further comprises cutting structures mountedthereon. These may be, for example, blades or the like, or sections ofhard facing material incorporated into the structure of the shoe.

The opening may also comprise portions of hard facing materialincorporated therein, to allow the opening to ream or cut sections ofthe bore or cuttings where necessary.

Preferably the opening further comprises a pin, bolt, or the likemounted therein, extending substantially perpendicular to the directionof the slot. This serves to hold the edges of the slot together, andprevent possible ‘flaring’ of the edges of the slot should the shoeencounter adverse conditions.

According to an aspect of the present invention there is provided acentraliser for mounting on a tubular member for location in a bore, thecentraliser comprising an annular body and a bearing for locationbetween the body and the tubular member.

Preferably, the bearing is formed to encourage thin film lubrication orformation of a hydrodynamic bearing and sacrificial self-lubrication inthe event that thin film lubrication or hydrodynamic bearing shouldbreak down.

According to a further aspect of the present invention there is provideda body for mounting on a string of tubular members coupled together byconnectors defining upsets in the string and for location in a bore, thebody having a tapering profile and being adapted for location on an endof a tubular member adjacent a connector, the taper leading fromadjacent the surface of the tubular member.

The provision of the tapered body assists in preventing the build up ofcuttings and other debris that often occurs at the connectors when astring of tubular members, such as a casing string, is run into adeviated or horizontal bore.

The body may have a maximum outer diameter corresponding to that of theconnector, or may define a larger outer diameter than the connector, toprovide a stand-off for the connector.

The body may define flutes, blades or pads, to facilitate bore cleaningor fluid flow past the body.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects of the present invention will now be described,by way of example only, and with reference to the accompanying drawings,in which:

FIG. 1 shows a centraliser in accordance with one embodiment of theinvention;

FIG. 2 shows a stop collar as may be used with the centraliser of FIG.1;

FIG. 3 shows a view of the stop collar of FIG. 2, with the parts of thestop separated;

FIG. 4 shows an alternative centraliser in combination with alternativestop collars;

FIG. 5 shows a bearing sleeve as may be used with centralisers inaccordance with embodiments of the present invention;

FIG. 6 shows a section of a portion of a centraliser in accordance withan embodiment of the present invention;

FIG. 7 shows a stop collar as may be used with centralisers inaccordance with embodiments of the present invention;

FIGS. 8 to 18 illustrate various alternative blade configurations as maybe used with a centraliser of the present invention;

FIGS. 19 to 21 illustrate further embodiments of a centraliser inaccordance with the present invention, arranged to provide a turbulentfluid flow in the bore and to provide rotation of the centraliser;

FIG. 22 shows a further alternative centraliser and stop collarcombination in accordance with an embodiment of the present invention;

FIG. 23 shows a body for mounting on a casing string in accordance witha further embodiment of the invention; and

FIGS. 24 and 25 show sectional and end views of a casing shoe inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first of all to FIG. 1, this shows a centraliser for mountingon a tubular, particularly casing, in accordance with an embodiment ofthe invention. The centraliser 10 comprises a cylindrical body 12, onwhich are mounted a plurality of blades 14. The body 12 in this exampleis made of steel, while the blades 14 are formed of a plastics material,such as Nylon 6.6. Alternatively, the blades may be formed homogeneouslywith the body, while the blades and/or the body may incorporate plasticor other low friction inserts or coating on or about the blades or body.Each blade is generally parallelogram-shaped, and stands proud of thesurface of the body. The spaces between the blades 14 define an unbrokenaxial and circumferential flow path for flow of mud, cement, and otherflowable preparations past the centraliser.

The centraliser 10 is provided in two sections which fit around a lengthof casing or drill pipe to enable the centraliser to be fitted andremoved without the need to be lifted over the end of the casing. Thesections may be provided with interlocking male and female members, or aretaining pin, in order to secure the centraliser on the casing or drillpipe.

The centraliser 10 is provided in combination with two stop collars, oneof which is illustrated in FIGS. 2 and 3. The stop collar 20 is mountedon a section of casing 22, and is comprised of two sections 24, 26. Theupper section 24 is provided with a series of deformable teeth 28 whichmay fit inside a tapered space 30 provided between the lip of the lowercollar section 26 and the casing 22. Co-operating male and femalethreads, serrations or profiles are provided on the outer surfaces ofthe teeth 28 and the inner surface of the lower collar section 26. Onfitting the collar 20 to a casing section or drill pipe, the twosections 24,26 are relatively rotated, pushed, or compressed to engagethe male and female threaded connections. As the sections are rotated,pushed, or compressed further, the tapered space 30 of the lower section26 forces the teeth 28 radially inwards to engage both the lower section26 and the surface of the casing 22. With sufficient tightening of thethreads, the stop collar 20 will be fixed with respect to the casing 22.

The centraliser 10 may be rotatably mounted on the casing above the stopcollar 20; a further stop collar may be located above the centraliser,in the opposite orientation to the collar illustrated in FIG. 3.

The lower edge 32 of the lower portion 26 of the collar 20 is tapered,as is the corresponding portion of the second collar; this eases theflow of fluid over and past the centraliser\collar arrangement, andfacilitates passage of the arrangement past ledges and otherobstructions.

An assembly of centraliser and collars is shown in FIG. 4. In thisillustration, the lower collar 42 is provided with aneccentrically-angled upper edge 44; the lower edge of the centraliser 46is correspondingly shaped. However, the upper collar 48 and the upperedge of the centraliser 46, are provided with co-operating edges, bothperpendicular to the casing axis. The collars may be fixed to the casingby means of set screws, bolts, dowels or the like; or by any othersuitable means.

As mentioned above, the collars 42, 48 are non-rotatable with respect tothe casing 50, while the centraliser 46 is normally rotatable. Thecentraliser 46 is also free to move axially with respect to the casing50, within the limits of the stops 42, 48.

As the casing or drill pipe is being lowered into the hole (that is,moving in the direction of arrow A), the centraliser 46 will moveupwards until it abuts the upper collar 48. Since the abutting edges areboth horizontal (assuming a vertical orientation of the casing), thecentraliser 46 will still be free to rotate relative to the casing 50and collar 48; the centraliser 46 will therefore remain stationaryrelative to the borehole walls if the casing is rotated, and will act todistance the casing 50 from the bore walls. Also, the abutting surfacesof the centraliser 46 and collar 48 are formed to facilitate relativerotation, the collar 48 defining a plane surface and the centraliser asemi-circular surface. If the casing encounters an obstacle while beingrun in to a bore, for example, a cuttings bed which restricts fluidcirculation and progress of the casing, the casing 50 may be raisedslightly in the opposite direction to arrow A. The centraliser 46 willthen move downward until it abuts the lower collar 42. The co-operatingedges of the collar 42 and centraliser 46 will interlock allowing thecentraliser 46 to be rotated with the casing 50. Thus, the blades 52 ofthe centraliser 46 will be rotating and scraping the bore wall, andthereby assist in dislodging the cuttings. It will be noted that theblades 52 are of slightly different configuration than those shown inFIG. 1.

Once the obstacle has been removed from the bore, the casing 50 may beadvanced into the bore once more, and the centraliser 46 will be free torotate relative to the casing 50.

In alternative arrangements, the relative positions of the stop collarsmay be reversed, so that the rotating and non-rotating directions ofdrilling are reversed also.

Although the collars 42, 48 are described as being non-rotating, theymay be arranged to rotate when subjected to torque, load, or force abovea certain level. For example, the teeth of the collar may be arranged toslip, shear or deform at certain torques, loads, or forces, so allowingrotation of the collar and centraliser preventing damage to the casing.

Collars 42, 48 may further be arranged to disengage into two or moreparts, with one part remaining fixed to the casing and the other being aloose bearing which is free to rotate, when subjected to torque, load,or force above a certain level and so allowing rotation of thecentraliser but preventing damage to the casing when overloaded. Thecollar may be formed by two parts held together by any suitable means,such as shear pins, glue, or the like, to slip, shear, or deform atcertain torques, loads, or forces, or may be one homogeneous part with ashear groove or notch machined which separates the stop screws and thecentraliser engaging means. Alternatively or in combination thereof thecollar may be formed in a material which is softer than the centraliser,and so will fail before the centraliser.

The collar/centraliser engagement may be configured in a variety of waysas to restrict relative rotation. This can be absolute, by way of squaretype/stepped/teeth arrangement, or relative, through an eccentric/sinewave/slip clutch type arrangement. Generally the centraliser will beconfigured to be able to engage and disengage from the collar. Howeverin some instances it may be preferable that the engagement is designedto be final, such that contact with overriding force will result in thecentraliser and stop collar becoming pressure fitted and rigidly andfirmly affixed to one another.

Although the centraliser may be mounted directly on the casing, relativerotation may abrade both the centraliser and the casing. For thisreason, a bearing sleeve 54 as illustrated in FIG. 5 may be mountedbetween the centraliser and casing. The sleeve 54 is a cylinder ofplastic or nylon which may be provided with a slit 56 to facilitatemounting over the casing. The bearing sleeve 54 provides sacrificiallubrication to the centraliser. Alternative bearing means may also orinstead be provided, for example, ball bearings, fluid film, and thelike.

An alternative method of securing a centraliser to the casing isillustrated in FIG. 6, which shows an enlarged sectional view of aportion of a centraliser. The centraliser 60 is mounted on a casing 62,and includes an annular recess 64 adjacent the casing 62, whichaccommodates a deformable annular member 66, the inner face 68 of whichis coated with a high friction material 70 (for example, an aggregate).The centraliser 60 is further provided with a number of Allen screws 72(only one shown) mounted in threaded bores, such that the tip of eachscrew 72 is in contact with the annular member, while the head of eachscrew 72 is recessed but accessible from the outside of the centraliser60. Set screws or the like may instead be used. The screws 72 areaccommodated by the thicker material present at the centraliser blades.Tightening of the screws 72 urges the annular member 66 against thecasing 62, so fixing the centraliser 60 to the casing.

A similar arrangement may be provided with stop collars as may be usedwith centralisers of certain embodiments of the invention, to permit orrestrict rotation as desired. Such a stop collar 74 is illustrated inFIG. 7. The collar 74 has an internal recess 76 in which a snap-ring ismounted, while a number of Allen screws 78 are mounted in thickenedportions 80 of the collar 74, in communication with the recess 76.

FIGS. 8 to 18 illustrate various different blade configurations whichmay be provided on the centraliser of embodiments of the presentinvention. Each blade arrangement has effects on the flow of fluids overthe centraliser and the cutting ability of the blades. For example,certain of the blades, for example, those illustrated in FIG. 8 bynumeral 14 a and FIG. 9 by 14 b, have recessed channels 15 a, 15 b,respectively, running along the long axis of the blade. These channelsallow cuttings and fluid to flow past the blade even while the blade iscutting, so improving the blade's ability to clean out a bore.Differently shaped features to perform such functions are shown at 15 cin blade 14 c in FIG. 10, at 15 d on blade 14 d in FIG. 11, at 15 e onblade 14 e in FIG. 12, and at 15 f on blade 14 f in FIG. 13. Stillother, differently shaped features to perform such functions are shownat 15 g on blade 14 g in FIG. 14 and at 15 h on blade 14 h in FIG. 15.

The blades 15 j shown in FIG. 16 have an outer surface coating of hardfacing, and are formed with an angled leading edge 14 j, so that thehard facing overhangs the base of the blade.

The arrangement of the blades 14 k shown in FIG. 18 provides aventuri-like flow across the centraliser 10 k; that is, the formation ofa constriction in the closed channel/flute 21 carrying the fluidincreases the velocity and kinetic energy of the fluid at the point ofconstriction, to promote turbulent fluid flow and to maximise jettingeffects in connection with mixing of the swept cutting bed particulatewithin the well bore fluids. Such a blade arrangement may be used withany of the other centralisers described herein.

FIG. 19 shows a centraliser according to the present invention with ablade configuration selected to provide a turbulent fluid flow over thecentraliser and to cause rotational force to be exerted on thecentraliser. It can be seen from the Figure that the two-part helicalblades 14 l of the centraliser 10 l are rectilinear on one side facethereof while the opposite side face curves outwards, and is generallyrounded. Other configurations may be straight-edged, provided the bladesgenerally form a fluid constriction with the circumferentially adjacentblade. This provides a channel between the blades which narrows,broadens, then narrows, as fluid passes upwards and over thecentraliser. The variation in channel size results in a change in fluidflow direction, speed and pressure as fluid flows upward between theblades. Once the fluid passes beyond the end of each blade part, thefluid speed drops, leading to turbulent fluid flow. The change in fluidflow causes the fluid to exert a generally lateral force on thecentraliser, so leading to rotation of the centraliser in the absence ofany countervailing force. This rotation causes any drill cuttings andthe like lying in the bore to be agitated and entrained in the fluidflow over the centraliser. Similarly, the turbulence of the flow overthe centraliser assists in carrying and entraining particulates and thelike along with the fluid, so preventing build up of these particulateson the centraliser or in the bore. This results in a cleaner bore andcentraliser than with conventional centraliser arrangements.

FIGS. 20 and 21 show an alternative centraliser arrangement to thatshown in FIG. 19, but which also provides for a turbulent fluid flow androtation of the centraliser. The centraliser 101 of FIG. 19 is madesubstantially from Austempered Ductile Iron, while that of FIGS. 20 and21 is made from plastics material. FIG. 21 shows a view of thecentraliser 10 m of FIG. 20 from above; it will be seen that the blades14 m of the centraliser 10 m are wrapped around the centraliser body,and that complete coverage of the circumference of the body is obtained.The centraliser 10 m functions in much the same manner as thecentraliser 101 of FIG. 19, to provide a turbulent fluid flow, alternateblade parts each having a rectilinear side face and a curved side face,and a rectilinear side face and a side face featuring a concave cut-out,which provides a “scooping” action if the centraliser is rotating.Alternatively, the blades may have straight side faces, provided thereis a change in blade width.

FIG. 22 shows a further alternative centraliser and stop collararrangement, in which both collars 20 n and the centraliser 10 n areprovided with mateable profiles in the form of co-operating waveformation surfaces. Various other mateable profile configurations may beused. The centraliser will normally rest at the centre of its range ofaxial movement, out of contact with either of the collars, and rotatablerelative to the casing. However, if the centraliser encounters anobstruction in the bore the centraliser will be urged against one of thecollars, depending on the direction of axial movement of the casing,thus causing the centraliser 10 n to rotate with the casing and causingthe blades 14 n to ream the bore wall.

FIG. 23 shows a body for mounting on a casing string in accordance witha further embodiment of the invention. Casing sections 90 are joinedtogether by tubular connectors 92 of larger bore than the casing 90. Thebody 94 of the invention has a tapering profile, and is mounted adjacentthe connector 92 such that the taper leads away from the connector 92.This assists in the flow of cuttings and other debris past the connector92. This aspect of the invention may, if desired, be combined withfeatures of the other embodiments described herein.

FIGS. 24 and 25 show side and end sectional views of a casing shoe inaccordance with an embodiment of the invention. The shoe 110 comprises abody 112 mounted on the end of a tubular section 114. The body 112carries a number of blades 116, each of which carries a coating of hardfacing material. A bore 118 extends through the body 112, leading to aslot-like opening 120 at the tip of the body 112. The opening 120 isalso surrounded by portions of hard facing material 122, and carries apin 124 mounted across the opening 120 perpendicular to the slot. Inuse, as the shoe 110 is advanced and rotated into a bore, the blades 116and sections of hard facing material 122 around the opening 120 ream orcut any obstructions and debris within the bore. Fluid may be pumpedalong the bore 118 within the shoe 110, which fluid leaves the opening120 and entrains cuttings and the like in its flow. This serves to carrycuttings and waste away from the end of the string, so preventingdeposition and accumulation of waste. The slot-like form of the opening120 means that should a particle of waste block a section of the opening120, fluid is still able to be pumped out from the opening 120 aroundthe obstruction. The bolt 124 across the opening 120 serves to hold theedges of the opening 120 together against any forces tending to splaythe opening 120 (for example, if the opening 120 does becomeobstructed), so reducing the likelihood of failure of the shoe 110.

It will be apparent to the skilled person that the foregoing is forillustrative purposes only, and that various modifications andvariations may be made to the apparatus described herein withoutdeparting from the scope of the invention. It is further envisaged thatany number of the above features may be combined and adapted for usewith a spring bow centraliser (that is, a centraliser which incorporatessprung blades). Although described herein primarily with reference tocasing sections, it will be apparent to the skilled person that theinvention may be used with other tubulars, such as drill pipe sections,or may be mounted on a mandrel for insertion into a drill string.

1. A centraliser used to facilitate the running in and cementing ofcasing, said centraliser comprising: a body adapted for mounting on acasing, said body defining a plurality of substantially rigid blades;and at least one stop for mounting on the casing, said at least one stophaving a first end carrying a means for selectively engaging, an end ofthe body to selectively restrict relative rotation between the body andthe casing, the at least one stop being mountable in a first orientationrelative to the body such that in the first orientation the first end ofthe stop is adapted to engage and rotate the body and is opposable tothe end of the body, and in a second orientation the first end of thestop does not engage with the body and is unopposable to the end of thebody.
 2. The centraliser of claim 1 wherein said at least one stopfurther comprises a second end located away from the body relative tothe first end which does not engage with the body to restrict relativerotation between the body and the casing when the second end of the stopis contact with the body.
 3. The centraliser of claim 1 wherein the atleast one stop is adapted to be fixed relative to the casing and thebody is adapted to be normally rotatable relative to the casing.
 4. Thecentraliser of claim 1 wherein the at least one stop is fixed to thecasing by set screws.
 5. The centraliser of claim 1 wherein the firstend of the stop and the end of the body define respective cooperatingformations which are adapted to selectively engage to restrict rotationtherebetween.
 6. The centraliser of claim 5 wherein the body is adaptedto be axially moveable relative to the casing such that the end of thebody is axially moveable into and out of engagement with the stop. 7.The centraliser of claim 6 wherein the stop and body are adapted andlocated such that axial movement of the casing through a bore in onedirection will tend to separate the stop and body, permitting rotationof the body relative to the casing, while movement of the casing in theopposite direction will tend to bring the stop and body together, suchthat the body is rotatable with the casing when the stop presents thefirst end to the end of the body.
 8. The centraliser of claim 1 whereinthe body further includes two stops, and the body is adapted formounting on the casing between the stops and configured such that thebody is selectively rotatable with the casing while the casing is beingrun into a bore and also while being retrieved from the bore.
 9. Thecentraliser of claim 1, wherein the body is nonrotatable relative to thecasing on experiencing an axial force in excess of a predeterminedlevel.
 10. The centraliser of claim 1, wherein the body is rotatablerelative to the casing on experiencing a torque above a predeterminedlevel.
 11. The centraliser of claim 1, wherein the blades are configuredto provide a stand-off between the casing and the bore wall and permitfluid circulation past the centraliser.
 12. The centraliser of claim 1wherein each adjoining pair of the plurality of blades is separated byat least one flute.
 13. The centraliser of claim 12 wherein at least oneof the blades and the respective flute are configured to induce arotational torque on the centraliser as fluid passes between the blades.14. The centraliser of claim 1 wherein the blades are configured toprovide a nozzle effect on fluid flowing between the blades.
 15. Thecentraliser of claim 14 wherein at least a portion of one of theplurality of blades is configured to taper along at least one part of alength of the centraliser.
 16. The centraliser of claim 1 wherein theblades are configured to provide for turbulent fluid flow betweenblades.
 17. The centraliser of claim 1 wherein the blades are configuredto taper along at least part of a length of the centraliser to provide achange in the velocity and kinetic energy of fluid flowing along thecentraliser.
 18. The centraliser of claim 1, further comprising abearing for engaging the casing.
 19. The centraliser of claim 18,wherein the bearing is configured so as to encourage thin filmlubrication between the centraliser and the casing.
 20. The centraliserof claim 19 wherein the bearing is configured so as to providesacrificial self-lubrication in the event that thin film lubricationshould break down.
 21. The centraliser of claim 1 wherein the blades areformed of a sacrificial self-lubricating material to minimize frictionbetween the centraliser and a bore wall.
 22. The centraliser of claim 1wherein the body is formed of polymeric material.
 23. The centraliser ofclaim 1 wherein the body is formed of metal.
 24. The centraliser ofclaim 1 wherein the body comprises a frame of relatively hard materialwhich is disposed on a portion of soft material.
 25. The centraliser ofclaim 1 wherein the body is formed of at least two parts configured soas to allow the body to be fitted around a casing.
 26. The centraliserof claim 1, wherein the at least one stop comprises at least two parts,such that when the parts are coupled together, a portion of at least oneof the parts is urged into engagement with the casing.
 27. Thecentraliser of claim 1, wherein the at least one stop comprises at leasttwo parts arranged to separate when subjected to a torque above apredetermined level.
 28. The centraliser of claim 1, wherein the atleast one stop comprises a body portion and a radially moveable grippingpart for selectively engaging the casing, and means for urging thegripping part into engagement with the casing.
 29. The centraliser ofclaim 1 wherein the at least one stop has a tapering leading face, tofacilitate movement over ledges and to prevent the build up of debris infront of the stop.
 30. The centraliser of claim 1 wherein the at leastone stop is formed of a softer material than the body.
 31. Thecentraliser of claim 1 wherein a wall thickness of the centraliservaries along a length of the centraliser to provide a change in thevelocity and kinetic energy of fluid flowing along the centraliser. 32.The centraliser of claim 1, wherein at least one blade comprises atleast two axially spaced blade portions.
 33. The centraliser of claim 1,wherein at least one blade comprises at least two axially spaced bladeportions, the blade portions being axially offset from one another. 34.The centraliser of claim 1, wherein the blades have cutting edges. 35.The centraliser of claim 1, wherein the body is substantially solid andthe blades are formed integrally of the body, and the blades define afixed diameter selected to be smaller than a bore into which the casingis run and cemented.
 36. A method of reaming a bore, comprising runninga casing having a bladed centraliser body and a stop disposed on thecasing, the stop having a first end carrying a means for selectivelyrotationally engaging the body into a previously drilled bore, andselectively rotationally coupling the body to the casing and rotatingthe body with the casing. thereby causing the body to ream the bore. 37.A method for running a casing, comprising: affixing selectivelyrotatably engageable centralizers to the exterior of the casing;inserting the casing into a wellbore; selectively engaging thecentralizers; rotating the casing with the engaged centralizers to reamthe wellbore; and continuing to insert the casing into the wellboreafter an obstruction is removed by the reaming.